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| Placing cooling fans in series for more CFM?? | |||
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| Posted by: riviron ® 04/03/2008, 15:57:39 Author Profile eMail author Edit |
Hello experts, I am a Mechanical Engineer and am trying to cool a heat sink but cannot get to a point where it is cool enough. The limitations are its enclosure size and and air outlet directions. Due to design limitations these cannot be changed. As usual one side of heat sink is flat(attached to heat source) and other side has fins to be cooled. So I can only use a 40X10 (max width) size 5V cooling fan due to the compactness of the design. Of course the closer the fan is to the fins' surface the cooler it gets but all fans have their circular hub in the centre which does not blow air in the centre. So a square shaped fan if brought close to the surface only blows donut shaped air current on the heat sink which will not cool the centre as much. In this case the heat source is right in the centre so the heat sink is much warmer in the centre compared to the corners. Well, the only thing I can play around with is the air flow rate which made significant difference in temperature while I was experimenting other designs. I have these fans available: 5V - 40X20mm - 9.5 CFM
MY QUESTION: If I put two 40X10mm fans back to back (along same axis AS SHOWN IN THE DIAGRAM) instead of one 40X20mm, would I get 1.5 or 2 times the CFM of 40X10mm?? OR Can I increase the CFM buy putting two 40X10mm fans just a little apart? If yes, apprx - what distance? OR What is the best way to increase CFM by any combination of these fans? OR If you could PLEASE comment on the proposed designed - will it work - if YES to what extent?? Thank you very much for your wise advice
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| : Placing cooling fans in series for more CFM?? | |||
| : Placing cooling fans in series for more CFM?? -- riviron | Post Reply | Top of thread | Forum |
| Posted by: jboggs ® 04/04/2008, 10:27:55 Author Profile eMail author Edit |
You seem concerned that the coooling fins were so much cooler than the heat source. I thought that was the purpose. The greater delta-T, the more BTU's are flowing, right? The core question to me is the actual limiting temperature of the heat source. If it is too close to the limit, and the fins are doing their job, which they seem to be, then the answer to me is to get the fins even cooler. Blowing more air on an area not designed to convect heat into it might not be as efficient. Bottom line, the low temperature of the fins is a non-issue compared to the high temperature of the source. |
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| : Placing cooling fans in series for more CFM?? -- riviron | Post Reply | Top of thread | Forum |
Posted by: randykimball ®  04/03/2008, 17:41:17 Author Profile eMail author Edit |
..or...
Can you drop the temerature of the ambient air being pumped in? The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. |
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| : : Placing cooling fans in series for more CFM?? -- randykimball | Post Reply | Top of thread | Forum |
| Posted by: CCR5600Design ® 04/04/2008, 09:12:40 Author Profile eMail author Edit |
Just a thought... If you used a small deflector between the fan and the heat sink, could you not direct some of the airflow from an area of higher flow to an area of lower flow to improve the efficiency of any fan used? I am not an engineer, so the mathematics and dynamics of the problem are not my forte, but it seems if you have ample airflow in one area, could you not divert a portion of it to an area of need to achieve your goal? Ron "What we need are more people who specialize in the impossible." - Theodore Roethke |
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| Posted by: riviron ® 04/04/2008, 11:21:48 Author Profile eMail author Edit |
Thanks for your thoughts but my question is not being diverted so may be I am not making it clear. Let me ask again: As shown in figure the heat source goes upto about 95 degrees C and it is supposed to be cooled to 50 degrees C. While cooling it comes back to about 70 degrees C but does not go below it. I am limited and restriced to change the size of enclosure or even heat sink. All I can do is change shapes, hole orientations and fans but they all should fit in the same enclosure and drop the temperature to 50 degrees C. The problem is that the contact area of heat source to heat sink is very small and right in the centre - the fan size is limited so of course CFM will be limited. If I increase heat sinks width then I will have to decrease fan width, if I put a curved plate between heat source and heat sink to dissipate heat evenly again I will have to reduce fan width. So I am trying to make sure if instead of one 20mm fan can I use two 10mm fans? Will they increase my CFM if I put them back to back? If they would increase then appx by what factor? As you can see the temp at centre is much higher than on edges, that is the reason for put something curved between heat source and heat sink so that it can spread the heat around on outerside. I tried to direct air in the centre but doing that reduces the velocity considerable and thus heats up the heat sink more so that does not work. No, I cannot drop the temp of ambient air. Yes great delta T better and more BTU flows but is there anyway to increase delta T just by changing design? Yes, the core question is how to cool or reduce the temp of heat source. So how can I transfer more heat from heat source to the other edge of fins so that it stays cooler? Does my idea of using two fans back to back instead of one big one seem feasible for increasing CFM? |
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| Posted by: jboggs ® 04/05/2008, 11:04:59 Author Profile eMail author Edit |
I have not designed electronic enclosures, but I do know that there are many different configurations of fans. Sounds like you need one that could produce the most volume from a long and thin cylindrical shape. Right? What other fan configurations are available? They don't all have to be the traditional axial flow design. |
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| Posted by: sneitzel ® 04/04/2008, 17:34:52 Author Profile eMail author Edit |
My response to the question of whether or not two fans in series increases flow: I don't think so. If both fans are the same size, speed, and blade pitch, then there won't be any increase in flow. The only way I could possibly imagine an increase in flow is if the air coming off of the first fan was not accelerated up to the velocity that it should have given the speed and pitch of the blades. Then it may be possible for the second fan to have an effect. A single fan that flows more would be the way to go. |
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| Posted by: zekeman ® 04/04/2008, 18:43:52 Author Profile eMail author Edit |
Why don't you state the heat source power in Watts, the ambient temperature and whether this is a steady state problem or not.
Ths two fan solution won't buy you much of an increase in flow,perhaps only about 20%. I would look for a centrifugal fan with a higher flow and would direct the flow, if possible, over the heat source first,and then over the fins. This would markedly improve the cooling . If you have the source power, then you can get an analytical determination of the cfm required, instead of guessing at a proper air flow. |
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| Posted by: zekeman ® 04/04/2008, 19:12:21 Author Profile eMail author Edit |
One more thing. The mfr stated cfm is usually for a free system, not where it flows into a constrained space, your situation. You won't get the cfm stated in your case. In order to determine cfm, you must have a mfr supplied fan curve and a system curve. |
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| Posted by: Bruised ® Bruiser, Bruiser 04/07/2008, 00:39:27 Author Profile eMail author Edit |
I did a heap of heatsink testing years ago and found that the air flow is not the big decider. You want turbulence, and a heatsink that has the right efficiency for the task. Based on your description, I'd suggest moving the fan back in the hope of getting rid of your 'donut' and promoting a turbulent, scrubbing effect. As for series/parallel fans, the rule for pumps is parallel for flow, series for pressure. |
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| Posted by: Bruised ® Bruiser, Bruiser 04/07/2008, 00:38:51 Author Profile eMail author Edit |
I did a heap of heatsink testing years ago and found that the air flow is not the big decider. You want turbulence, and a heatsink that has the right efficiency for the task. Based on your description, I'd suggest moving the fan back in the hope of getting rid of your 'donut' and promoting a turbulent, scrubbing effect. As for series/parallel fans, the rule for pumps is parallel for flow, series for pressure. |
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